The present invention generally relates to pilot symbol allocation in wireless communication systems, and particularly relates to adaptive allocation of pilot symbols based on changing channel conditions.
In a wireless communication system, known symbols referred to as common pilot symbols are transmitted across a wireless communication channel to receiving devices. The receiving devices use the common pilot symbols for estimating channel response which in turn is used to coherently demodulate received data symbols. For example, in an Orthogonal Frequency Division Multiplexing (OFDM) system, common pilot symbols are transmitted across the time-frequency plane. An OFDM receiving device estimates the time-frequency response of the channel based on the common pilot symbols in order to perform coherent data symbol demodulation. Since the time-frequency response of an OFDM channel is a slow-varying two-dimensional process, the common pilot symbols essentially sample this process and therefore need to have a density that is high enough for receiving devices to reconstruct (or interpolate) the full response.
The minimum density required for aliasing-free reconstruction of a communication channel's response is referred to as the Nyquist rate in sampling theory. The Nyquist rate is inversely proportional to the channel's maximum delay-Doppler spread. Because common pilot symbols occupy the radio resource that otherwise could be used for data transmission, pilot symbol overhead is kept as low as possible. This is especially critical for advanced cellular communication systems in which a large number of base stations with multiple antennas may be simultaneously visible to a terminal. Large pilot signal overhead can significantly limit the radio resource available for data transmission.
Common pilot signal density is conventionally set at about twice the Nyquist rate to cover worst-case channel conditions. Correspondingly, conventional wireless communication systems transmit common pilot symbols at a fixed spacing corresponding to approximately twice the Nyquist rate while inserting data symbols between the pilot symbols. For example, common pilot signal density is conventionally set at about twice the Nyquist rate in both the time and frequency domains for OFDM communication systems. For a mobile system operating at a carrier frequency of 5 GHz, the maximal Doppler spread ( υ) and delay spread ( τ) are given by: υ≈2000(±1000) Hz, τ≈7.8125 μsec  (1)where the maximal Doppler and delay spreads correspond to a vehicle speed of 200 kmph and a maximum scatterer spread over 2343.75 meters.
In order to meet the time domain Nyquist sampling criterion, one common pilot symbol is transmitted every 1/ υ seconds. Since the duration of the common pilot symbol Ts is just the symbol length of the OFDM system, there can be at most 1/( υTs) OFDM data symbols between two consecutive common pilot symbols. Similarly, the number of sub-carriers between two consecutive common pilot symbols in the frequency domain can be at most Ts/ τ (ignoring cyclic prefix). Therefore, the minimum common pilot signal density is υτ, which is 1/64 for the parameters given above.
When common pilot symbols are transmitted at twice the Nyquist rate in both the time and frequency domains, the pilot signal density for the typical OFDM environment described above is approximately 1/16. The situation worsens as carrier frequency and cell size increase. For a terminal that is in the coverage area of four base stations, 25% of the radio resource is conventionally occupied by the common pilot signal. This estimate does not account for multiple antennas that may be used to transmit common pilot symbols for spatial multiplexing systems such as Multiple Input Multiple Output (MIMO) systems.
When the common pilot signal density is only marginally above the Nyquist rate, the pilot observation window may have to be extended beyond the data transmission, resulting in delay that may not be acceptable for certain applications. Furthermore, a common pilot signal having a fixed density is very inefficient for an environment that has a large variation in the delay-Doppler spreads experienced by different users. A fixed, high-density common pilot signal benefits users only in extreme cases such as when a user is moving at a very high rate of speed or experiences a highly dispersive channel. All other users do not require a high-density common pilot signal to accurately estimate channel response, and thus, bandwidth is unnecessarily consumed by transmitting unneeded pilot symbols instead of data symbols.